Infrared Spectroscopy of Epitaxial Antimony Sulpho Iodide Thin Films
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Infrared Spectroscopy of Epitaxial Antimony Sulpho Iodide Thin Films S. Kotru, S. Surthi and R. K. Pandey Department of Electrical and Computer Engineering The University of Alabama, Tuscaloosa, AL 35487-0286, U.S.A. D. Donnelly Department of Physics, Southwest Texas State University, San Marcos, TX
ABSTRACT Thin films of antimony sulpho iodide (SbSI) were grown on platinized silicon substrates by the pulsed laser deposition method. As grown films were amorphous and annealing at 250 °C for 5 minutes introduced crystallinity in the films. Infrared reflectance measurements were done in the frequency range ~ 500 – 5000 cm-1 (wavelength ~ 2–20 µm). The reflectance measurements were taken at temperatures above and below the ferroelectric transition of SbSI ~ 20° C. The index of refraction for a (121) oriented film was determined to be 2.83 ± 0.35 at a temperature of 25.6 °C, and 2.80 ± 0.35 at a temperature of 9.6 °C. For a (002) oriented film, the index was 3.82 ± 0.48 at a temperature of 26.5 °C, and 3.76 ± 0.48 at a temperature of 8.0 °C. Pyro-optic coefficients of 1.5 x 10-3 °C -1 for the (121) oriented film, and 3.2 x 10-3 °C -1 for the (002) oriented film were obtained. These results are consistent with measurements done in the visible region, and demonstrate the potential of SbSI as an infrared detector material.
INTRODUCTION Antimony sulpho iodide (SbSI) is one of the ternary chalcogonides of the group of A B CVII compounds (where A = Sb, Bi; B = S, Se, Te; and C = I, Cl, Br). It crystallizes in an orthorhombic structure with lattice constants: a = 8.49 Å, b = 10.1 Å, and c = 4.16 Å, c-axis being the polar axis. The crystal structure is shown in Figure 1, where three unit cells are shown in paraelectric phase. This material has been extensively studied in the single crystal form. It has been established as a semiconducting material with band gap of 1.97 eV [1] as well as a ferroelectric material with Curie temperature (TC) ~ 20 °C [2]. Single crystals have very high values of piezoelectric [3], pyroelectric [4] and pyro-optic coefficients [5]. In addition SbSI is also photo-ferroelectric, its absorption edge lies in the red and near-infrared region, and its ferroelectric properties change when optically excited. It has the highest value of refractive index (4.5) of any known material. The refractive index changes considerably when the E-field of the incident polarized light is parallel to the c-axis [6]. Temperature changes of the order of 10-3 °C can be detected near the TC (15-19 °C) by monitoring the reflectance coefficient of SbSI, which is more sensitive than the best values for single element pyroelectric devices [5]. All these properties make SbSI a potential candidate for an infrared detector for thermal imaging. V
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In spite of all these interesting properties, this material has not yet seen any practical device development. The reason is the difficulty in growing device quality single crystals or films of this material due to a large inherent growth anisotropy. We have been successful in C7.14.1
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